Showing papers on "Circular dichroism published in 2018"
TL;DR: It is shown that the orientation and twisting of β-sheets account for the observed spectral diversity, and a new method to estimate accurately the secondary structure is developed, and the originality of BeStSel is that it carries out a detailed secondary structure analysis.
Abstract: Circular dichroism (CD) spectroscopy is a widely used method to study the protein secondary structure. However, for decades, the general opinion was that the correct estimation of β-sheet content is challenging because of the large spectral and structural diversity of β-sheets. Recently, we showed that the orientation and twisting of β-sheets account for the observed spectral diversity, and developed a new method to estimate accurately the secondary structure (PNAS, 112, E3095). BeStSel web server provides the Beta Structure Selection method to analyze the CD spectra recorded by conventional or synchrotron radiation CD equipment. Both normalized and measured data can be uploaded to the server either as a single spectrum or series of spectra. The originality of BeStSel is that it carries out a detailed secondary structure analysis providing information on eight secondary structure components including parallel-β structure and antiparallel β-sheets with three different groups of twist. Based on these, it predicts the protein fold down to the topology/homology level of the CATH protein fold classification. The server also provides a module to analyze the structures deposited in the PDB for BeStSel secondary structure contents in relation to Dictionary of Secondary Structure of Proteins data. The BeStSel server is freely accessible at http://bestsel.elte.hu.
666 citations
01 May 2018
394 citations
TL;DR: The results show that CD spectra of nucleic acids can be analyzed to obtain quantitative structural information about secondary structure content in an analogous way to methods used to analyze proteinCD spectra.
Abstract: A curated library of circular dichroism spectra of 23 G-quadruplexes of known structure was built and analyzed. The goal of this study was to use this reference library to develop an algorithm to derive quantitative estimates of the secondary structure content of quadruplexes from their experimental CD spectra. Principal component analysis and singular value decomposition were used to characterize the reference spectral library. CD spectra were successfully fit to obtain estimates of the amounts of base steps in anti-anti, syn-anti or anti-syn conformations, in diagonal or lateral loops or in other conformations. The results show that CD spectra of nucleic acids can be analyzed to obtain quantitative structural information about secondary structure content in an analogous way to methods used to analyze protein CD spectra.
288 citations
TL;DR: It was found that dispersions and gels of paramagnetic Co3O4 nanoparticles with chiral distortions of the crystal lattices exhibited chiroptical activity in the visible range that was 10 times as strong as that of nonparamagnetic nanoparticles of comparable size.
Abstract: Chiral inorganic nanostructures have high circular dichroism, but real-time control of their optical activity has so far been achieved only by irreversible chemical changes. Field modulation is a far more desirable path to chiroptical devices. We hypothesized that magnetic field modulation can be attained for chiral nanostructures with large contributions of the magnetic transition dipole moments to polarization rotation. We found that dispersions and gels of paramagnetic Co3O4 nanoparticles with chiral distortions of the crystal lattices exhibited chiroptical activity in the visible range that was 10 times as strong as that of nonparamagnetic nanoparticles of comparable size. Transparency of the nanoparticle gels to circularly polarized light beams in the ultraviolet range was reversibly modulated by magnetic fields. These phenomena were also observed for other nanoscale metal oxides with lattice distortions from imprinted amino acids and other chiral ligands. The large family of chiral ceramic nanostructures and gels can be pivotal for new technologies and knowledge at the nexus of chirality and magnetism.
170 citations
TL;DR: The potential use of circularly polarized luminescence for object identification in a sensor application is demonstrated and chemical stimuli-responsibility allow sensing using the CPL signal as detection output, enabling to discriminate between a signal from the target analyte and that from non-target species.
Abstract: The potential use of circularly polarized luminescence for object identification in a sensor application is demonstrated. New luminescence probes using pyrene derivatives as sensor luminophores were developed. (R,R)-Im2 Py and (S,S)-Im2 Py contain two chiral imidazole moieties at 1,6-positions through ethynyl spacers (angle between spacers ca. 180°). The probe molecules spontaneously self-assemble into chiral stacks (P or M helicity) upon coordination to metal ions with tetrahedral coordination (Zn2+ ). The chiral probes display neither circular dichroism (CD) nor circularly polarized luminescence (CPL) without metal ions. However, (R,R)-Im2 Py and (S,S)-Im2 Py exhibit intense chiroptical activity (CD and CPL) upon self-assembly with Zn2+ ions. (R,R)-Im2 Py and (S,S)-Im2 Py with chemical stimuli-responsibility allow sensing using the CPL signal as detection output, enabling us to discriminate between a signal from the target analyte and that from non-target species.
143 citations
TL;DR: In this article, photoexcitation circular dichroism was proposed and demonstrated to be an order of magnitude more sensitive than photoabsorption circular dichromychroism in neutral molecules, taking advantage of the coherent helical motion of bound electrons excited by ultrashort circularly polarized light.
Abstract: Chiral effects appear in a wide variety of natural phenomena and are of fundamental importance in science, from particle physics to metamaterials. The standard technique of chiral discrimination—photoabsorption circular dichroism—relies on the magnetic properties of a chiral medium and yields an extremely weak chiral response. Here, we propose and demonstrate an orders of magnitude more sensitive type of circular dichroism in neutral molecules: photoexcitation circular dichroism. This technique does not rely on weak magnetic effects, but takes advantage of the coherent helical motion of bound electrons excited by ultrashort circularly polarized light. It results in an ultrafast chiral response and the efficient excitation of a macroscopic chiral density in an initially isotropic ensemble of randomly oriented chiral molecules. We probe this excitation using linearly polarized laser pulses, without the aid of further chiral interactions. Our time-resolved study of vibronic chiral dynamics opens a way to the efficient initiation, control and monitoring of chiral chemical change in neutral molecules at the level of electrons.
141 citations
TL;DR: A simple method to monitor in-situ and in real-time the conformational change of (R)-1,1′-binaphthyl-based polymers during the aggregation process using circular dichroism is demonstrated.
Abstract: The aggregation of molecules plays an important role in determining their function. Electron microscopy and other methods can only characterize the variation of microstructure, but are not capable of monitoring conformational changes. These techniques are also complicated, expensive and time-consuming. Here, we demonstrate a simple method to monitor in-situ and in real-time the conformational change of (R)-1,1′-binaphthyl-based polymers during the aggregation process using circular dichroism. Based on results from molecular dynamics simulations and experimental circular dichroism measurements, polymers with “open” binaphthyl rings are found to show stronger aggregation-annihilated circular dichroism effects, with more negative torsion angles between the two naphthalene rings. In contrast, the polymers with “locked” rings show a more restrained aggregation-annihilated circular dichroism effect, with only a slight change of torsion angle. This work provides an approach to monitor molecular aggregation in a simple, accurate, and efficient way. Molecular aggregation is a widespread and important process in physiological metabolism, but details regarding conformational changes during the process are hard to probe. Here, the authors use circular dichroism to monitor in-situ the conformational changes occurring during molecular aggregation.
124 citations
TL;DR: This study proposes a new type of chiral spectroscopy-photothermal circular dichroism that is much greater than the equivalent effect in any chiral molecular system or plasmonic bioassembly and potential applications are in polarization-sensitive surface photochemistry and chiral bolometers.
Abstract: Chiral photochemistry remains a challenge because of the very small asymmetry in the chiro-optical absorption of molecular species However, we think that the rapidly developing fields of plasmonic chirality and plasmon-induced circular dichroism demonstrate very strong chiro-optical effects and have the potential to facilitate the development of chiral photochemistry and other related applications such as chiral separation and sensing In this study, we propose a new type of chiral spectroscopy–photothermal circular dichroism It is already known that the planar plasmonic superabsorbers can be designed to exhibit giant circular dichroism signals in the reflection Therefore, upon illumination with chiral light, such planar metastructures should be able to generate a prominent asymmetry in their local temperatures Indeed, we demonstrate this chiral photothermal effect using a chiral plasmonic absorber Calculated temperature maps show very strong photothermal circular dichroism One of the structures com
121 citations
TL;DR: The sensitive and selective detection of a target RNA sequence from the hepatitis C virus genome is shown and a reconfigurable DNA origami template has been used where a chiral arrangement of gold nanorods on the structure can lead to the generation of strong circular dichroism (CD).
Abstract: Detecting small sequences of RNA in biological samples such as microRNA or viral RNA demands highly sensitive and specific methods. Here, a reconfigurable DNA origami template has been used where a chiral arrangement of gold nanorods on the structure can lead to the generation of strong circular dichroism (CD). Switching of the cross-like DNA structure is achieved by the addition of nucleic acid sequences, which arrests the structure in one of the possible chiral states by specific molecular recognition. A specific sequence can thus be detected through the resulting changes in the plasmonic CD spectrum. We show the sensitive and selective detection of a target RNA sequence from the hepatitis C virus genome. The RNA binds to a complementary sequence that is part of the lock mechanism, which leads to the formation of a defined state of the plasmonic system with a distinct optical response. With this approach, we were able to detect this specific RNA sequence at concentrations as low as 100 pm.
104 citations
TL;DR: From both experimental measurements and theoretical considerations, it is concluded that the transferred CD is most intensive for systems with strong plasmonic hotspots, as they find them in relatively small gaps between spherical nanoparticles and preferably between the tips of nanorods.
Abstract: The chiral state of a molecule plays a crucial role in molecular recognition and biochemical reactions. Because of this and owing to the fact that most modern drugs are chiral, the sensitive and reliable detection of the chirality of molecules is of great interest to drug development. The majority of naturally occurring biomolecules exhibit circular dichroism (CD) in the UV range. Theoretical studies and several experiments have demonstrated that this UV-CD can be transferred into the plasmonic frequency domain when metal surfaces and chiral biomolecules are in close proximity. Here, we demonstrate that the CD transfer effect can be drastically enhanced by placing chiral molecules, here double-stranded DNA, inside a plasmonic hotspot. By using different particle types (gold, silver, spheres, and rods) and by exploiting the versatility of DNA origami, we were able to systematically study the impact of varying particle distances on the CD transfer efficiency and to demonstrate CD transfer over the whole optical spectrum down to the near-infrared. For this purpose, nanorods were also placed upright on DNA origami sheets, forming strong optical antennas. Theoretical models, demonstrating the intricate relationships between molecular chirality and achiral electric fields, support our experimental findings. From both experimental measurements and theoretical considerations, we conclude that the transferred CD is most intensive for systems with strong plasmonic hotspots, as we find them in relatively small gaps (5-12 nm) between spherical nanoparticles and preferably between the tips of nanorods.
96 citations
TL;DR: The successful preparation of l- and d-cysteine-capped CdSe-dot/CdS-rods (DRs) with tunable CD and CPL behaviors is described and the molar ratio of chiral cysteine to the DRs is investigated.
Abstract: Ligand-induced chirality in semiconductor nanocrystals (NCs) has attracted attention because of the tunable optical properties of the NCs. Induced circular dichroism (CD) has been observed in CdX (X = S, Se, Te) NCs and their hybrids, but circularly polarized luminescence (CPL) in these fluorescent nanomaterials has been seldom reported. Herein, we describe the successful preparation of l- and d-cysteine-capped CdSe-dot/CdS-rods (DRs) with tunable CD and CPL behaviors and a maximum anisotropic factor ( glum) of 4.66 × 10-4. The observed CD and CPL activities are sensitive to the relative absorption ratio of the CdS shell to the CdSe core, suggesting that the anisotropic g-factors in both CD and CPL increase to some extent for a smaller shell-to-core absorption ratio. In addition, the molar ratio of chiral cysteine to the DRs is investigated. Instead of enhancing the chiral interactions between the chiral molecules and DRs, an excess of cysteine molecules in aqueous solution inhibits both the CD and CPL activities. Such chiral and emissive NCs provide an ideal platform for the rational design of semiconductor nanomaterials with chiroptical properties.
TL;DR: This tutorial aims to help researchers entering the research field to organize experiments accurately and to interpret the obtained data reliably as a tool for the characterization of adducts between nucleic acids and small ligands.
Abstract: The structural characterization of non-covalent complexes between nucleic acids and small molecules (ligands) is of a paramount significance to bioorganic research. Highly informative methods about nucleic acid/ligand complexes such as single crystal X-ray diffraction or NMR spectroscopy cannot be performed under biologically compatible conditions and are extensively time consuming. Therefore, in search for faster methods which can be applied to conditions that are at least similar to the naturally occurring ones, a set of polarization spectroscopy methods has shown highly promising results. Electronic circular dichroism (ECD) is the most commonly used method for the characterization of the helical structure of DNA and RNA and their complexes with ligands. Less common but complementary to ECD, is flow-oriented linear dichroism (LD). Other methods such as vibrational CD (VCD) and emission-based methods (FDCD, CPL), can also be used for suitable samples. Despite the popularity of polarization spectroscopy in biophysics, aside several highly focused reviews on the application of these methods to DNA/RNA research, there is no systematic tutorial covering all mentioned methods as a tool for the characterization of adducts between nucleic acids and small ligands. This tutorial aims to help researchers entering the research field to organize experiments accurately and to interpret the obtained data reliably.
TL;DR: A chiral, shape-persistent, perylene-diimide-based nanoribbon based on a helical superstructure with two [6]helicene subcomponents exhibits very intense electronic circular dichroism, including one of the largest Cotton effects ever observed in the visible range.
Abstract: We report the synthesis and characterization of a chiral, shape-persistent, perylene-diimide-based nanoribbon. Specifically, the fusion of three perylene-diimide monomers with intervening naphthalene subunits resulted in a helical superstructure with two [6]helicene subcomponents. This π-helix-of-helicenes exhibits very intense electronic circular dichroism, including one of the largest Cotton effects ever observed in the visible range. It also displays more than an order of magnitude increase in circular dichroism for select wavelengths relative to its smaller homologue. These impressive chiroptical properties underscore the potential of this new nanoribbon architecture in the context of chiral electronic materials.
TL;DR: In this paper, a grafting reaction was performed between dextran and the cysteine (Cys) and lysine (Lys) from 12S globulin, which greatly improved the solubility, emulsifying property, and thermal stability of the conjugates.
TL;DR: A series of chiral fluorescent macrocycles display a remarkable combination of both +/– ECD and strong on/off CPL reversible switching upon cation binding and displacement.
Abstract: Straightforward synthesis and resolution of a series of chiral fluorescent macrocycles are presented, together with their electronic circular dichroism (ECD), strong excimer fluorescence (EF, λ 300 to 650 nm) and allied highly circularly polarized luminescence (CPL, glum up to 1.7 × 10−2). The ECD, EF and CPL responses are strongly affected by the presence of metal ions (Na+, Ba2+) thanks to deep conformational changes. While ECD signals can be almost completely reversibly inverted upon the complexation/decomplexation of metal ions in a typical binary response, CPL signals are reversibly quenched concomitantly. The designed macrocycles display thus a remarkable combination of both +/− ECD and on/off CPL reversible switching.
TL;DR: Egg white protein was phosphorylated with microwave assistance in the presence of sodium tripolyphosphate at pH 8.0, and the structural and foaming properties of the protein were investigated in this paper.
Abstract: Egg white protein was phosphorylated with microwave assistance (200 W, 300 W, 400 W, 500 W and 600 W) in the presence of sodium tripolyphosphate at pH 8.0, and the structural and foaming properties of the protein were investigated in this work. The degree of phosphorylation increased at first and then remained unchanged. Fourier Transform infrared spectroscopy showed that egg white had new characteristic absorption bands by phosphorylation. Circular dichroism spectroscopy and surface hydrophobicity analysis revealed that the phosphorylated proteins had a more unfolded and flexible structure. The phosphorylated proteins had higher absolute values of zeta potential and a lower content of free -SH. After treatment with microwave at the 500 W level, phosphorylated protein had a 0.44 times (P
TL;DR: The principles and experimental design of FPOP that enable its fast labeling are summarized and the current and unique capabilities of the technique in protein higher order structure elucidation are described, providing high spatial resolution with residue-specific information.
Abstract: ConspectusAssessment of protein structure and interaction is crucial for understanding protein structure/function relationships Compared to high-resolution structural tools, including X-ray crystallography, nuclear magnetic resonance (NMR), and cryo-EM, and traditional low-resolution methods, such as circular dichroism, UV–vis, and florescence spectroscopy, mass spectrometry (MS)-based protein footprinting affords medium-to-high resolution (ie, regional and residue-specific insights) by taking advantage of proteomics methods focused on the primary structure The methodology relies on “painting” the reactive and solvent-exposed amino acid residues with chemical tags and using the pattern of modifications as footprints from analysis by bottom-up MS-based proteomics to deduce protein higher order structures The outcome can refer to proteins in solution or even in cells and is complementary to those of X-ray crystallography and NMR It is particularly useful in mapping protein–ligand interfaces and confor
TL;DR: The temperature- and concentration-dependent circular dichroism spectra indicate that the chiro-optical response arises from chiral imprinting by the ligand on the electronic states of the quantum-confined perovskite rather than chiral ligand-induced stereoselective aggregation.
Abstract: The direct synthesis of chiroptical organic-inorganic methylammonium lead bromide perovskite nanoplatelets that are passivated by R- or S-phenylethylammonium ligands is reported. The circular dichroism spectra can be divided into two components: (1) a region associated with a charge transfer transition between the ligand and the nanoplatelet, 300-350 nm, and (2) a region corresponding to the excitonic absorption maximum of the perovskite, 400-450 nm. The temperature- and concentration-dependent circular dichroism spectra indicate that the chiro-optical response arises from chiral imprinting by the ligand on the electronic states of the quantum-confined perovskite rather than chiral ligand-induced stereoselective aggregation.
TL;DR: This work demonstrates the first chiral inversion induced by combination of metal ion and anion and presents a new perspective on the supramolecular coordination chemistry of pillar[ n]arenes.
Abstract: Chiral inversion of single molecules has been a challenging task because chirality information controls structures and functions of various molecules, artificial nanostructures, DNA, and proteins Herein we present a pseudo[1]catenane-type molecule whose planar chiral inversion is driven by a metal ion under the control of anions for the first time Considering an in–out equilibrium of a fused thiacrown and the soft metal binding, pillar[5]thiacrown (rac-L) was synthesized Two planar-chiral enantiomers of rac-L (in-pS-L and in-pR-L) were isolated and the absolute configuration was determined by circular dichroism and single crystal X-ray analysis The in-pS-L recognizes Hg2+ to trigger the chiral inversion to out-pR-L, to our surprise; it takes place only in the presence of ClO4– or NO3– among the anions used In the mercury(II) perchlorate complex solution, anion-exchange from ClO4– to I– or removal of Hg2+ by addition of S2– makes the system reversible The crystallographic approach reveals that the an
TL;DR: HA-cNGs loaded with saporin, a ribosome inactivating protein, exhibited significantly enhanced apoptotic activity to MCF-7 cells with a low IC50 of 12.2 nM, and have appeared as a simple and multifunctional platform for efficient intracellular protein delivery.
TL;DR: Racemization studies show high stability (no racemization at 70 °C) of these chiral nanoclusters, which hold great promise in applications such as asymmetry catalysis.
Abstract: A general method, using mixed ligands (here diphosphines and thiolates) is devised to turn an achiral metal cluster, Au13 Cu2 , into an enantiomeric pair by breaking (lowering) the overall molecular symmetry with the ligands. Using an achiral diphosphine, a racemic [Au13 Cu2 (DPPP)3 (SPy)6 ]+ was prepared which crystallizes in centrosymmetric space groups. Using chiral diphosphines, enantioselective synthesis of an optically pure, enantiomeric pair of [Au13 Cu2 ((2r,4r)/(2s,4s)-BDPP)3 (SPy)6 ]+ was achieved in one pot. Their circular dichroism (CD) spectra give perfect mirror images in the range of 250-500 nm with maximum anisotropy factors of 1.2×10-3 . DFT calculations provided good correlations with the observed CD spectra of the enantiomers and, more importantly, revealed the origin of the chirality. Racemization studies show high stability (no racemization at 70 °C) of these chiral nanoclusters, which hold great promise in applications such as asymmetry catalysis.
TL;DR: In this article, the change in structure of ovalbumin (OVA) was determined by spectroscopy of Fluorescence, Circular Dichroism and Fourier-Transform Raman.
TL;DR: CDtoolX as mentioned in this paper is a free software program that enables processing, displaying, archiving, calibrating, comparisons, and analyses of CD and synchrotron radiation circular dichroism spectroscopic data.
Abstract: Circular dichroism (CD) spectroscopy is a highly used method for the examination and characterization of proteins, including, amongst other features, their secondary and tertiary structures, thermal stability, comparisons of wildtype and mutant proteins, and monitoring the binding of small molecules, folding/unfolding pathways, and formation of macromolecular complexes. This article describes CDtoolX, a new, user-friendly, free-to-download-and-use software program that enables processing, displaying, archiving, calibrating, comparisons, and analyses of CD and synchrotron radiation circular dichroism spectroscopic data.
TL;DR: In this article, a chiral plasmonic hydrogen-sensing platform using palladium-based nanohelices is demonstrated and the linearity of the circular dichroism in response to hydrogen is significantly improved.
Abstract: In this article, a chiral plasmonic hydrogen-sensing platform using palladium-based nanohelices is demonstrated. Such 3D chiral nanostructures fabricated by nanoglancing angle deposition exhibit strong circular dichroism both experimentally and theoretically. The chiroptical properties of the palladium nanohelices are altered upon hydrogen uptake and sensitively depend on the hydrogen concentration. Such properties are well suited for remote and spark-free hydrogen sensing in the flammable range. Hysteresis is reduced, when an increasing amount of gold is utilized in the palladium-gold hybrid helices. As a result, the linearity of the circular dichroism in response to hydrogen is significantly improved. The chiral plasmonic sensor scheme is of potential interest for hydrogen-sensing applications, where good linearity and high sensitivity are required.
TL;DR: A simple and versatile method to increase the stickiness of BSA protein molecules adsorbing onto silica surfaces, resulting in up to a 10-fold improvement in blocking efficiency against serum biofouling.
Abstract: Bovine serum albumin (BSA) is the most widely used protein for surface passivation applications, although it has relatively weak, nonsticky interactions with hydrophilic surfaces such as silica-based materials Herein, we report a simple and versatile method to increase the stickiness of BSA protein molecules adsorbing onto silica surfaces, resulting in up to a 10-fold improvement in blocking efficiency against serum biofouling Circular dichroism spectroscopy, dynamic light scattering, and nanoparticle tracking analysis showed that temperature-induced denaturation of BSA proteins in bulk solution resulted in irreversible unfolding and protein oligomerization, thereby converting weakly adhesive protein monomers into a more adhesive oligomeric form The heat-treated, denatured BSA oligomers remained stable after cooling Room-temperature quartz crystal microbalance-dissipation and localized surface plasmon resonance experiments revealed that denatured BSA oligomers adsorbed more quickly and in larger mass
TL;DR: Fluorescence spectra showed that the fluorescence quenched in HSA was mainly contributed to by tryptophan residues, and the atrazine bound to HSA, which induced changes in the conformation and secondary structure of HSA and caused an energy transfer.
Abstract: The herbicide atrazine is widely used across the globe, which is a great concern. To investigate its potential toxicity in the human body, human serum albumin (HSA) was selected as a model protein. The interaction between atrazine and HSA was investigated using steady-state fluorescence spectroscopy, synchronous fluorescence spectroscopy, UV-Vis spectroscopy, three-dimensional (3D) fluorescence spectroscopy and circular dichroism (CD) spectroscopy. The intrinsic fluorescence of HSA was quenched by the atrazine through a static quenching mechanism. Fluorescence spectra at two excitation wavelengths (280 and 295 nm) showed that the fluorescence quenched in HSA was mainly contributed to by tryptophan residues. In addition, the atrazine bound to HSA, which induced changes in the conformation and secondary structure of HSA and caused an energy transfer. Thermodynamic parameters revealed that this binding is spontaneous. Moreover, electrostatic interactions play a major role in the combination of atrazine and HSA. One atrazine molecule can only bind to one HSA molecule to form a complex, and the atrazine molecule is bound at site II (subdomain IIIA) of HSA. This study furthers the understanding of the potential effects posed by atrazine on humans at the molecular level.
TL;DR: In this paper, the effect of the interaction between anthocyanins and β-casein on the stability was investigated by fluorescence quenching technique and circular dichroism spectroscopy.
TL;DR: Molecular chirality and the inherently connected differential absorption of circular polarized light (CD) combined with semiconducting properties offers great potential for chiral opto-electronics and this work provides a new small molecular benchmark material for the development of organic thin film based chiroptics.
Abstract: Molecular chirality and the inherently connected differential absorption of circular polarized light (CD) combined with semiconducting properties offers great potential for chiral opto-electronics. Here we discuss the temperature-controlled assembly of enantiopure prolinol functionalized squaraines with opposite handedness into intrinsically circular dichroic, molecular J-aggregates in spincasted thin films. By Mueller matrix spectroscopy we accurately probe an extraordinary high excitonic circular dichroism, which is not amplified by mesoscopic ordering effects. At maximum, CD values of 1000 mdeg/nm are reached and, after accounting for reflection losses related to the thin film nature, we obtain a film thickness independent dissymmetry factor g = 0.75. The large oscillator strength of the corresponding absorption within the deep-red spectral range translates into a negative real part of the dielectric function in the spectral vicinity of the exciton resonance. Thereby, we provide a new small molecular benchmark material for the development of organic thin film based chiroptics.
TL;DR: In this paper, a unified description of several methods of chiral discrimination based exclusively on electric-dipole interactions is presented, including photoelectron-photoexcitation circular dichroism (PXECD), enantiosensitive microwave spectroscopy (EMWS), and photoexcitation cyclic dichroisms (PCD).
Abstract: We present a unified description of several methods of chiral discrimination based exclusively on electric-dipole interactions. It includes photoelectron circular dichroism (PECD), enantiosensitive microwave spectroscopy (EMWS), photoexcitation circular dichroism (PXCD), and photoelectron-photoexcitation circular dichroism (PXECD). We show that, in spite of the fact that the physics underlying the appearance of a chiral response is very different in all these methods, the enantiosensitive and dichroic observable in all cases has a unique form. It is a polar vector given by the product of (i) a molecular pseudoscalar and (ii) a field pseudovector specified by the configuration of the electric fields interacting with the isotropic ensemble of chiral molecules. The molecular pseudoscalar is a rotationally invariant property, which is composed from different molecule-specific vectors and in the simplest case is a triple product of such vectors. The key property that enables the chiral response is the noncoplanarity of the vectors forming such triple product. The key property that enables chiral detection without relying on the chirality of the electromagnetic fields is the vectorial nature of the enantiosensitive observable. Our compact and general expression for this observable shows what ultimately determines the efficiency of the chiral signal and if, or when, it can reach 100%. We also discuss the differences between the two phenomena, which rely on the bound states, PXCD and EMWS, and the two phenomena using the continuum states, PECD and PXECD. Finally, we extend these methods to arbitrary polarizations of the electric fields used to induce and probe the chiral response.
TL;DR: In this article, structural changes and allergenicity of whey proteins (WPI) upon complexation with caffeic acid (CA) and (-)-epigallocatechin gallate (EGCG) at acidic and neutral pH conditions were investigated.